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Simulators >> Circuit Simulators >> high-Q crystal oscillator simulation time
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Message started by Visjnoe on Dec 3^rd, 2006, 6:20am

Title: high-Q crystal oscillator simulation time
Post by Visjnoe on Dec 3^rd, 2006, 6:20am

Hey,

it is generally stated that high-Q (crystal) oscillators take a long time to simulate due to their high Q.
Could anyone please explain this from the simulator's (SPICE) internal working/algorithms?

Kind Regards

Peter

Title: Re: high-Q crystal oscillator simulation time
Post by makelo on Dec 11^th, 2006, 7:41am

The lumped resonant model for the crystal increases the size of the inductor and decreases the size of the capacitor as the Q increases. It is typical to have an inductor with several milliHenries with a fempto-farad capacitor. This creates some very slow time constants before achieving steady state.
One strategy is to adjust your crystal model (smaller L, larger C) to a lower Q ~100 for initial simulations and raise it back later.

Title: Re: high-Q crystal oscillator simulation time
Post by simon2 on Sep 20^th, 2007, 5:03pm

Hi Visjnoe,
it actually has nothing to do with the simulator, but everything to do with the real-world circuit (which the simulator should accurately reflect): in simple terms, the build up of the envelope is directly related to the damping (and hence "Q") of the circuit - if it is a "high Q" circuit it takes a long time ....

The converse of this is that by examining two points of the oscillation's envelope before limiting, you can directly calculate "loaded Q" for that oscillator.

Cheers,
SimonH.

Title: Re: high-Q crystal oscillator simulation time
Post by sheldon on Sep 21^st, 2007, 5:58am

Peter,

Simulating a crystal oscillator can take a long time to simulate in the
time domain. As Simon pointed out due to the high Q, it can take a long
time to reach steady state. However, frequency domain simulation of
crystal oscillators is very fast and this is how they are generally simulated.

Best Regards,

Sheldon